Vehicle turning behavior control apparatus

ABSTRACT

A vehicle turning behavior control apparatus for use with a multi-wheel automatic vehicle. The apparatus provides a difference between the braking forces applied to the nearside and offside wheels of the vehicle to produce a yaw moment in a direction to which the vehicle turns. The difference is determined based upon the vehicle steering angle and is modified based upon the vehicle steering speed.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for controlling the behavior of amulti-wheeled automotive vehicle turning either to the left or to theright and, more particularly, to such a vehicle turning behavior controlapparatus for providing different braking forces to the offside andnearside wheels of the vehicle.

For example, Japanese Utility Model Kokai No. 59-155264 discloses ananti-skid braking apparatus arranged to improve the initial vehicleheading performance by delaying the time at which brakes are applied tothe offside wheels of the vehicle so as to produce a yaw moment in adirection correcting an understeer characteristic when the vehiclesteering angle exceeds a predetermined value. However, such an anti-skidbraking apparatus fails to provide a good vehicle turning behaviorcontrol meeting to the driver's demand during a transient conditionwhere the steering wheel is turning. For example, the conventionalanti-skid braking apparatus operates at a slow response rate and the yawrate will rise somewhat behind an actual steering angle change when thedriver turns the steering wheel at a high speed.

SUMMARY OF THE INVENTION

Therefore, it is a main object of the invention to provide a vehicleturning behavior control apparatus which can operate at a rapid responserate with respect to steering angle changes so as to provide an improvedvehicle turning behavior control during transient conditions.

There is provided, in accordance with the invention, an apparatus forcontrolling turning behavior of a multi-wheeled automotive vehiclesupported on a plurality of pairs of wheels. The apparatus comprisesbraking means for applying braking forces to the respective wheels,first sensor means sensitive to a vehicle steering condition forproducing a first signal indicative of a sensed vehicle steeringcondition, second sensor means sensitive to a vehicle steering speed forproducing a second signal indicative of a sensed vehicle steering speed,and a control unit coupled to the first and second sensor means. Thecontrol unit includes means for providing a difference between thebraking forces applied to nearside and offside wheels of at least one ofthe pairs of wheels based upon the sensed vehicle steering condition toproduce a yaw moment in a direction to which the vehicle turns, andmeans for modifying the difference based upon the sensed vehiclesteering speed to increase the difference as the vehicle steering speedincreases.

In another aspect of the invention, the vehicle turning behavior controlapparatus comprises braking means for applying braking forces to therespective wheels, first sensor means sensitive to a vehicle steeringangle for producing a first signal indicative of a sensed vehiclesteering angle, second sensor means sensitive to a vehicle steeringspeed for producing a second signal indicative of a sensed vehiclesteering speed, and a control unit coupled to the first and secondsensor means. The control unit includes means for calculating a basicvalue ΔP1 as a function of the sensed vehicle steering angle to increasethe basic value as the sensed vehicle steering angle increases when thesensed vehicle steering angle exceeds a predetermined value, means forcalculating a first correction factor K1 as a function of the sensedvehicle steering speed to increase the first correction factor as thesensed vehicle steering speed increases, means for calculating adifference ΔP as ΔP=ΔP1×K1, and means for setting the braking means toprovide the calculated difference ΔP between the braking forces appliedto nearside and offside wheels of at least one of the pairs of wheels sothat the braking force applied to the offside wheel is smaller than thebraking force applied to the nearside wheel.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described in greater detail by reference to thefollowing description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a schematic diagram of a vehicle turning behavior controlapparatus embodying the invention;

FIG. 2 is a flow diagram showing the programming of the digital computerused in the vehicle turning behavior control apparatus;

FIG. 3 is a graph of vehicle steering angle θ versus fluid pressuredifference ΔP1;

FIG. 4 is a graph of vehicle steering speed θ' versus first correctionfactor K1;

FIG. 5 is a graph of vehicle speed V versus second correction factor K2;and

FIG. 6 is a flow diagram showing a modified form of the programming ofthe digital computer used in the vehicle turning behavior controlapparatus.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, and in particular to FIG. 1, there isshown a schematic diagram of a vehicle turning behavior controlapparatus embodying the invention. The invention will be described inconnection with an automotive vehicle supported on a pair of frontwheels 1L and 1R spaced laterally apart at equal distances from avehicle longitudinal axis and a pair of rear wheels 2L and 2R spacedlaterally apart at equal distances from the vehicle longitudinal axis.The numeral 3 designates a brake pedal which actuates the pistons in atandem master cylinder 4 and forces fluid through a first hydraulicsystem to wheel cylinders 5L and 5R situated in front wheel brakes forapplication of brakes to the respective front wheels 1L and 1R and alsothrough a second hydraulic system to wheel cylinders 6L and 6R situatedin rear wheel brakes for application of brakes to the respective rearwheels 2L and 2R.

The first hydraulic system includes a pressure responsive change-overvalve 8F which has an inlet connected through a conduit 7F to the mastercylinder 4 and an outlet connected through a pilot cylinder 9F to aconduit 10F. The change-over valve 8F normally occupies in a firstposition, illustrated in FIG. 1, to provide communication between themaster cylinder 4 and the pilot cylinder 9F. The change-over valve 8Fresponds to a fluid pressure by changing to a second position permittingfluid flow from the master cylinder 4 to the pilot cylinder 9F but notvice versa. The pilot cylinder 9F includes a piston 9c provided forreciprocating movement within the pilot cylinder valve bore to defineoutput and input chambers 9a and 9b on the opposite sides of the piston9c. The piston 9c is urged toward the illustrated first position bymeans of a compression spring 9d placed within the pilot cylinder valvebore. When the input chamber 9b receives a fluid pressure, the piston 9cmoves against the resilient force of the compression spring 9d to forcefluid from the output chamber 9a to the conduit 10F. The conduit 10F isdivided into two conduits 11F and 12F, the first conduit 11F beingconnected through a pressure control valve 13F to the wheel cylinder 5Lprovided for the left front wheel 1L. The second conduit 12F isconnected through another pressure control valve 14F to the wheelcylinder 5R provided for the right front wheel 1R. A pump 20F isprovided to supply fluid from a reservoir 19F to the conduit 10F when itis running and an accumulator 21F is charged up the fluid flow throughthe conduit 10F. The pressure control valve 13F operates on a currentsignal i1 fed thereto to occupy one of three positions. The firstposition, illustrated in FIG. 1, is occupied when the current signal i1has a level of zero amperes and the pressure control valve 13F providescommunication of the conduit 11F with the wheel cylinder 5L provided forthe left front wheel 1L so as to increase the fluid pressure introducedto the wheel cylinder 5L. The second position is encountered when thecurrent signal i1 has a level of two amperes and the pressure controlvalve 13F interrupts the communication between the conduit 11F and thewheel cylinder 5L so as to retain the fluid pressure in the wheelcylinder 5L. The third position is encountered when the current signali1 has a level of five amperes and the pressure control valve 13Fprovides communication between the wheel cylinder 5L and the reservoir19F so as to reduce the fluid pressure in the wheel cylinder 5L. Thepressure control valve 14F operates on a current signal i2 fed theretoto occupy one of three positions. The first position, illustrated inFIG. 1, is occupied when the current signal i2 has a level of zeroamperes and the pressure control valve 14F provides communication of theconduit 11F with the wheel cylinder 5R provided for the right frontwheel 1R so as to increase the fluid pressure introduced to the wheelcylinder 5R. The second position is encountered when the current signali2 has a level of two amperes and the pressure control valve 14Finterrupts the communication between the conduit 11F and the wheelcylinder 5R so as to retain the fluid pressure in the wheel cylinder 5R.The third position is encountered when the current signal i2 has a levelof five amperes and the pressure control valve 14F providescommunication between the wheel cylinder 5R and the reservoir 19F so asto reduce the fluid pressure in the wheel cylinder 5R. The pump 20F runswhen the pressure control valves 13F and 14F are in the second or thirdposition.

Similarly, the second hydraulic system includes a pressure responsivechange-over valve 8R which has an inlet connected through a conduit 7Rto the master cylinder 4 and an outlet connected through a pilotcylinder 9R to a conduit 10R. The change-over valve 8R normally occupiesin a first position, illustrated in FIG. 1, to provide communicationbetween the master cylinder 4 and the pilot cylinder 9R. The change-overvalve 8R responds to a fluid pressure by changing to a second positionpermitting fluid flow from the master cylinder 4 to the pilot cylinder9R but not vice versa. The pilot cylinder 9R includes a piston 9cprovided for reciprocating movement within the pilot cylinder valve boreto define output and input chambers 9a and 9b on the opposite sides ofthe piston 9c. The piston 9c is urged toward the illustrated firstposition by means of a compression spring 9d placed within the pilotcylinder valve bore. When the input chamber 9b receives a fluidpressure, the piston 9c moves against the resilient force of thecompression spring 9d to force fluid from the output chamber 9a to theconduit 10R. The conduit 10R is divided into two conduits 11R and 12R,the first conduit 11R being connected through a pressure control valve13R to the wheel cylinder 6L provided for the left rear wheel 2L. Thesecond conduit 12R is connected through another pressure control valve14R to the wheel cylinder 6R provided for the right rear wheel 2R. Apump 20R is provided to supply fluid from a reservoir 19R to the conduit10R when it is running and a accumulator 21R is charged up the fluidflow through the conduit 10R. The pressure control valve 13R operates ona current signal i3 fed thereto to occupy one of three positions. Thefirst position, illustrated in FIG. 1, is occupied when the currentsignal i3 has a level of zero amperes and the pressure control valve 13Rprovides communication of the conduit 11R with the wheel cylinder 6Lprovided for the left rear wheel 2L so as to increase the fluid pressureintroduced to the wheel cylinder 6L. The second position is encounteredwhen the current signal i3 has a level of two amperes and the pressurecontrol valve 13R interrupts the communication between the conduit 11Rand the wheel cylinder 6L so as to retain the fluid pressure in thewheel cylinder 6L. The third position is encountered when the currentsignal i3 has a level of five amperes and the pressure control valve 13Rprovides communication between the wheel cylinder 6L and the reservoir19R so as to reduce the fluid pressure in the wheel cylinder 6L. Thepressure control valve 14R operates on a current signal i4 fed theretoto occupy one of three positions. The first position, illustrated inFIG. 1, is occupied when the current signal i4 has a level of zeroamperes and the pressure control valve 14R provides communication of theconduit 11R with the wheel cylinder 6R provided for the right rear wheel2R so as to increase the fluid pressure introduced to the wheel cylinder6R. The second position is encountered when the current signal i4 has alevel of two amperes and the pressure control valve 14R interrupts thecommunication between the conduit 11R and the wheel cylinder 6R so as toretain the fluid pressure in the wheel cylinder 6R. The third positionis encountered when the current signal i4 has a level of five amperesand the pressure control valve 14R provides communication between thewheel cylinder 6R and the reservoir 19R so as to reduce the fluidpressure in the wheel cylinder 6R. The pump 20R runs when the pressurecontrol valves 13R and 14R are in the second or third position.

The pressure responsive change-over valves 8F and 8R and the pilotcylinders 9F and 9R are controlled by an automatic braking systemincluding a solenoid change-over valve 18. The solenoid change-overvalve 18 has three ports. The first port connected to the pressureresponsive change-over valves 8F and 8R and also to the input chambers9b of the respective pilot cylinders 9F and 9R. The second port isconnected to a reservoir 16 from which a pump 15 supplies fluid to thethird port of the solenoid change-over valve 18. An accumulator 17 ischarged up the fluid flow to the third port of the solenoid change-overvalve 18. The solenoid change-over valve 18 operates on a current signali5 fed thereto to occupies one of two positions. The first position,illustrated in FIG. 1, is occupied when the current signal i5 has alevel of zero amperes for the purpose of operating the braking system ina foot braking mode and the solenoid change-over valve 18 providescommunication between its first and second ports. In the first position,thus, no fluid pressure is applied to the change-over valves 8F and 8Rand the pilot cylinders 9F and 9R. Consequently, the fluid pressuredischarged from the pilot cylinders 9F and 9R to the respective conduits10F and 10R depends upon the degree of depression of the brake pedal 3.The second position is encountered when the current signal i5 has alevel of two amperes for the purpose of operating the braking system inan automatic braking mode and the solenoid change-over valve 18 providescommunication between its first and third ports. In the second position,thus, a constant fluid pressure is applied to change the change-overvalves 8F and 8R to the second positions preventing back-flows from thepilot cylinders 9F and 9R to the master cylinder 4. The constant fluidpressure is also introduced into the input chambers 9b of the pilotcylinders 9F and 9R to force fluid from the output chambers 9a to therespective conduits 10F and 10R. Consequently, the fluid pressuredischarged from the pilot cylinders 9F and 9R does not depend upon thedegree of depression of the brake pedal 3 and it depends upon the fluidpressure from the accumulator 17.

The levels of the respective current signals i1, i2, i3, i4 and i5 arerepetitively determined from calculations performed by an control unit22, these calculations being made based upon various conditions of theautomotive vehicle that are sensed during its operation. These sensedconditions include steering angle θ, brake pedal depression, wheelspeeds Vw1, Vw2, Vw3 and Vw4, vehicle lateral acceleration g, andsteering speed θ'. Thus, a steering angle sensor 23, a brake switch 24,wheel speed sensors 25, 26, 27 and 28, and a lateral acceleration sensor29 are connected to the control unit 22.

The steering angle sensor 23 is provided to sense the degree θ ofrotation of the steering wheel with respect to its neutral position andit produces an electric signal indicative of the sensed steering angleθ. The steering angle indication signal has a sign indicating thedirection to which the vehicle steering handle turns. The brake switch24 is associated with the brake pedal 3 to close to supply current fromthe engine battery to the control unit 22 in response to the applicationof foot braking to the vehicle (when the brake pedal 3 is depressed).The wheel speed sensors 25, 26, 27 and 28 are provided to sense theperipheral speeds Vw1, Vw2, Vw3 and Vw4 of rotation of the respectivewheels 1L, 1R, 2L and 2R and they produces electrical signals indicativeof the sensed wheel speeds Vw1, Vw2, Vw3 and Vw4. The lateralacceleration sensor (lateral G sensor) 29 senses the vehicle lateralacceleration g and produces an electric signal indicative of the sensedlateral acceleration g. An appropriate vehicle steering speed sensor maybe provided for producing an electric signal indicative of a sensedvehicle steering speed θ'. In the illustrated embodiment, the vehiclesteering speed θ' is calculated based upon the sensed vehicle steeringangle. This calculation will be described later in greater detail.

The numeral 31L, 31R, 32L and 32R designates fluid pressure sensorsprovided to sense the fluid pressures P1, P2, P3 and P4 introduced intothe respective wheel cylinders 5L, 5R, 6L and 6R. The fluid pressuresensors produces electric signals indicative of the sensed fluidpressures P1, P2, P3 and P4 to the control unit 22. The control unit 22employs these fluid pressure indication signals for providing feedbackcontrols of the fluid pressures introduced into the respective wheelcylinders 5L, 5R, 6L and 6R.

The control unit 22 employs the signals from the wheel speed sensors 25,26, 27 and 28 to provide anti-skid and traction controls. For a tractioncontrol, the control unit 22 outputs a control signal to an engineoutput controller.

When the brake pedal 3 is depressed, the control unit 22 receives asignal indicative of this condition from the brake switch 24 and setsthe current signal i5 at zero amperes to hold the solenoid change-overvalve 18 in the first (off) position, illustrated in FIG. 1. As aresult, the change-over valves 8F and 8R and the pilot cylinders 9F and9R are held in the illustrated positions. The control unit 23 sets thecurrent signals i1 to i4 at zero amperes to hold the pressure controlvalves 13F, 14F, 13R and 14R in the positions, illustrated in FIG. 1,unless the wheels are locked by the brakes. Consequently, the fluidpressures supplied into the respective wheel cylinders 5L, 5R, 6L and 6Rare substantially proportional to the force applied by the driver to thebrake pedal 3.

The control unit 22 provides an anti-skid control. For this purpose, thecontrol unit 22 employs the wheel speeds Vw1, Vw2, Vw3 and Vw4 tocalculate a pseudo vehicle speed in a well-known manner. The controlunit 22 employs the wheel speeds Vw1, Vw2, Vw3 and Vw4 to calculatebraking slip factors for use in determining the brake lock conditionsfor the respective wheels. When there is a tendency toward a brake lockcondition for one of the wheels, the control unit 22 sets thecorresponding one of the current signals i1, i2, i3 and i4 at twoamperes to change the corresponding pressure control valve to the secondposition retaining the fluid pressure in the corresponding wheelcylinder. If a brake lock condition occurs for the wheel, the controlunit 22 sets the current signal at five amperes to change thecorresponding control valve to the third position decreasing the fluidpressure in the corresponding wheel cylinder.

The control unit 22 provides a vehicle turning behavior control. Forthis purpose, the control unit 22 calculates a basic value ΔP1 as afunction of the vehicle steering angle θ to increase the basic value ΔP1as the vehicle steering angle θ increases, a correction factor K1 as afunction of the vehicle steering speed θ' to increase the correctionfactor K1 as the vehicle steering speed θ' increases, and a differenceΔP as ΔP=ΔP1×K1. The control unit 22 sets the pressure control valves13F, 14F, 13R and 14R to provide the calculated difference ΔP betweenthe braking forces applied to the nearside and offside wheels of thevehicle. The offside wheel is subject to a smaller braking force thanthe nearside wheel so as to produce a yaw moment in a direction to whichthe vehicle turns.

The control unit 22 employs a digital computer including a centralprocessing unit (CPU), a random access memory (RAM), a read only memory(ROM), and an input/output control unit (I/O). The central processingunit communicates with the rest of the computer via data bus. The readonly memory contains the program for operating the central processingunit and further contains appropriate data in look-up tables used incalculating appropriate values for the drive current signals i1 to i5.

FIG. 2 is a flow diagram illustrating the programming of the digitalcomputer. The computer program is entered at the point 102 at uniformintervals of time. At the point 104 in the program, the sensed vehiclesteering angle θ, and the sensed fluid pressures P1, P2, P3 and P4 areread into the random access memory. At the point 106 in the program, avehicle steering speed θ' is calculated as θ'=θ-θold where θ is thevehicle steering speed value read in the present cycle of execution ofthe program and θold is the vehicle steering speed value read in thelast cycle of execution of the program. Since this program is entered atuniform intervals of time, the difference (θ-θ') represents a rate ofchange of the vehicle steering angle. At the point 108 in the program, adetermination is made as to whether or not the brake switch 24 is turnedon. This determination is made based upon the current signal fed fromthe brake switch 24. If the answer to this question is "yes", then itmeans that the driver depresses the brake pedal 3 and the programproceeds to the point 110 for providing a vehicle turning behaviorcontrol.

At the point 110 in the program, a fluid pressure difference value ΔP iscalculated as a function of vehicle steering angle θ and vehiclesteering speed θ'. For this purpose, the central processing unitcalculates a basic fluid pressure difference value ΔP1 from arelationship programmed into the computer. This relationship specifiesthe basic fluid pressure difference value ΔP1 as a function of steeringangle θ. One example of such a relationship is shown in FIG. 3 where thebasic fluid pressure difference value ΔP1 is zero when the steeringangle θ is equal to or less than a predetermined value θ1 and itincreases as the steering angle θ increases when the steering angle θ isgreater than the predetermined value θ1. A correction factor K1 is usedto modify the calculated basic fluid pressure difference value ΔP1 so asto obtain the fluid pressure difference value ΔP as ΔP=K1×ΔP1. Thecorrection factor K1 is calculated from a relationship programmed intothe computer. This relationship defines the correction factor K1 as afunction of vehicle steering speed θ '. One example of such arelationship is shown in FIG. 4 where the correction factor K1 increasesas the vehicle steering speed θ' increases toward a predeterminedmaximum value (for example, 1.0). The calculated fluid pressuredifference value ΔP corresponds to a difference between the fluidpressures to be introduced into the wheel cylinders 5L and 5R providedfor the respective front wheels 1L and 1R and also to the differencebetween the fluid pressures to be introduced into the wheel cylinders 6Land 6R provided for the respective rear wheels 2L and 2R.

At the point 112 in the program, target values P1L, P1R, P2L and P2R forthe fluid pressures to be introduced into the wheel cylinders 5L, 5R, 6Land 6R provided for the respective wheels 1L, 1R, 2L and 2R arecalculated. These target values are calculated in such a manner that asmaller braking force is applied to an outer or offside one of the frontwheels positioned on the outside of a circle in which the vehicle movesthan is applied to the other inner or nearside front wheel and a smallerbraking force is applied to an outer or offside one of the rear wheelspositioned on the outside of the circle than is applied to the otherinner or nearside rear wheel. It is to be understood that the directionto which the cornering path is curved is determined based upon the signof the signal from the steering angle sensor 23.

For example, when the vehicle is turning to the left, the right frontwheel (offside wheel) 1R is subject to a smaller braking force than theleft front wheel (nearside wheel) 1L and the right rear wheel (offsidewheel) 2R is subject to a smaller braking force than the left rear wheel(nearside wheel) 2L. For this purpose, the central processing unitcalculates a target value P1L for the fluid pressure to be introducedinto the wheel cylinder 5L provided for the left front wheel 1L asP1L=P1 where P1 is the sensed fluid pressure introduced into the wheelcylinder 5L and a target value P1R for the fluid pressure to beintroduced into the wheel cylinder 5R provided for the right front wheel1R as P1R=P1-ΔP. The central processing unit further calculates a targetvalue P2L for the fluid pressure to be introduced into the wheelcylinder 6L provided for the left rear wheel 2L as P2L=P3 where P3 isthe sensed fluid pressure introduced into the wheel cylinder 6L and atarget value P2R for the fluid pressure to be introduced into the wheelcylinder 6R provided for the right rear wheel 2R as P2R=P3-ΔP.

When the vehicle is turning to the right, the left front wheel 1L(offside wheel) is subject to a smaller braking force than the rightfront wheel (nearside wheel) 1R and the left rear wheel (offside wheel)2L is subject to a smaller braking force than the right rear wheel(nearside wheel) 2R. For this purpose, the central processing unitcalculates a target value P1R for the fluid pressure to be introducedinto the wheel cylinder 5R as P1R=P2 where P2 is the sensed fluidpressure introduced into the wheel cylinder 5R and a target value P1Lfor the fluid pressure to be introduced into the wheel cylinder 5Lprovided for the left front wheel 1L as P1L=P2-ΔP. The centralprocessing unit further calculates a target value P2R for the fluidpressure to be introduced into the wheel cylinder 6R as P2R=P4 where P4is the sensed fluid pressure introduced into the wheel cylinder 6R and atarget value P2L for the fluid pressure to be introduced into the wheelcylinder 6L provided for the left rear wheel 2L as P2L=P4-ΔP.

At the point 116 in the program, the central processing unit calculatestarget values for the current signals i1, i2, i3 and i4 applied to therespective pressure control valves 13F, 14F, 13R and 14R. Thesecalculations are made based upon the calculated target values P1L, P1R,P2L and P2R. At the point 116 in the program, the calculate targetcurrent signal values are transferred to the input/output control unit.The input/output control unit sets the current signals i1, i2, i3 and i4to cause the pressure control valves 13F, 14F, 13R and 14R to controlthe fluid pressures to the wheel cylinders 5L, 5R, 6L and 6R to thecalculated target values P1L, P1R, P2L and P2R, respectively. In theillustrated case, the input/output control unit maintains the currentsignals i1 and i3 as they stand and takes the current signals i2 and i4in the form of on-off signals to reduce the fluid pressures P2 and P4when the vehicle moves along a left-handed cornering path and itmaintains the current signals i2 and i4 as they stand and takes thecurrent signals i1 and i3 in the form of on-off signals to reduce thefluid pressures P1 and P3 when the vehicle moves along a right-handedcornering path. The input/output control unit employs the fluid pressuresensors to provide feedback controls of the fluid pressures P1, P2, P3and P4. After the calculated values are transferred to the input/outputcontrol unit, the program proceeds to the end point 120.

If the answer to the question inputted at the point 108 is "no", then itmeans that the brake pedal 3 is released and the program proceeds to thepoint 118 where the target fluid pressure value ΔP is set at zero.Following this, the program proceeds to the point 112. In this case, thecontrol unit 23 provide no vehicle turning behavior control when thebrake pedal 3 is released.

Although the fluid pressure difference value ΔP has been described ascalculated as a function of steering angle θ and steering speed θ', itis to be understood that the fluid pressure difference value ΔP may becalculated as a function of steering angle θ, steering speed θ' andvehicle speed V. In this case, the fluid pressure difference value ΔP iscalculated as ΔP=K1×K2×ΔP1 where K2 is a correction factor calculatedfrom a relationship which specifies the correction factor K2 as afunction of vehicle speed V. One example of such a relationship is shownin FIG. 5 where the correction factor K2 decreases from 1.0 as thevehicle speed V increases. The vehicle speed V may be calculated basedupon the speeds of the driven wheels of the vehicle.

When the brake pedal 3 is depressed to apply a braking force to each ofthe wheels of the vehicle turning either to the right or to the left,the braking forces applied to the offside wheels are smaller than thosecorresponding to the brake pedal force. As a result, the vehicle issubject to a yaw moment in the direction to which the vehicle is turningso as to promote the tendency of the vehicle to turn in the direction.If the braking force difference is determined as a function of steeringangle, the vehicle turning behavior control has a slow response ratewith respect to a steering angle change when the steering wheel isturned at a high speed. According to this embodiment, the differencebetween the braking forces applied to the offside and inside wheels iscorrected for the vehicle steering speed θ' in such a manner that thebraking force difference increases as the vehicle steering speed θ'increases. As a result, the yaw moment increases as the rate θ' ofchange of the vehicle steering angle θ increases.

Although the vehicle turning condition is detected based upon thevehicle steering angle θ, it is to be understood that the yaw rate orthe lateral acceleration g sensed by the lateral acceleration sensor 29may be used singly or in combination with the vehicle steering angle θ.

FIG. 6 is a flow diagram illustrating a modified form of the programmingof the digital computer. The computer program is entered at the point202 at uniform intervals of time. At the point 204 in the program, thesensed steering angle θ and the sensed fluid pressures P1, P2, P3 and P4are read into the random access memory. At the point 206 in the program,a vehicle steering speed θ' is calculated as θ'=θ-θold where θ is thevehicle steering speed value read in the present cycle of execution ofthe program and θold is the vehicle steering speed value read in thelast cycle of execution of the program. Since this program is entered atuniform intervals of time, the difference (θ-θold) represents a rate ofchange of the vehicle steering angle. At the point 208 in the program, adetermination is made as to whether or not the brake switch 24 is turnedon. This determination is made based upon the current signal fed fromthe brake switch 24. If the answer to this question is "yes", then itmeans that the driver depresses the brake pedal 3 and the programproceeds to the point 210 for providing a vehicle turning behaviorcontrol.

At the point 210 in the program, a fluid pressure difference value ΔP iscalculated as a function of steering angle θ and steering speed θ'. Forthis purpose, the central processing unit calculates a basic fluidpressure difference value ΔP1 from a relationship programmed into thecomputer. This relationship specifies the basic fluid pressuredifference value ΔP1 as a function of steering angle θ. One example ofsuch a relationship is shown in FIG. 3 where the basic fluid pressuredifference value ΔP1 is zero when the steering angle θ is equal to orless than a predetermined value θ1 and it increases as the steeringangle θ increases when the steering angle θ is greater than thepredetermined value θ1. A correction factor K1 is used to modify thecalculated basic fluid pressure difference value ΔP1 so as to obtain thefluid pressure difference value ΔP as ΔP=K1×ΔP1. The correction factorK1 is calculated from a relationship programmed into the computer. Thisrelationship defines the correction factor K1 as a function of steeringspeed θ'. One example of such a relationship is shown in FIG. 4 wherethe correction factor K1 increases as the steering speed θ' increases.The calculated fluid pressure difference value ΔP corresponds to adifference between the fluid pressures to be introduced into the wheelcylinders 5L and 5R provided for the respective front wheels 1L and 1Rand also to the difference between the fluid pressures to be introducedinto the wheel cylinders 6L and 6R provided for the respective rearwheels 2L and 2R.

At the point 214 in the program, target values P1L, P1R, P2L and P2R forthe fluid pressures to be introduced into the wheel cylinders 5L, 5R, 6Land 6R provided for the respective wheels 1L, 1R, 2L and 2R arecalculated. These target values are calculated in such a manner that asmaller braking force is applied to an outer or offside one of the frontwheels positioned on the outside of a circle in which the vehicle movesthan is applied to the other inner or nearside front wheel and a smallerbraking force is applied to an outer or offside one of the rear wheelspositioned on the outside of the circle than is applied to the otherinner or nearside rear wheel. It is to be understood that the directionto which the cornering path is curved is determined based upon the signof the signal from the steering angle sensor 23.

For example, when the vehicle is turning to the left, the right frontwheel (offside wheel) 1R is subject to a smaller braking force than theleft front wheel (nearside wheel) 1L and the right rear wheel (offsidewheel) 2R is subject to a smaller braking force than the left rear wheel(nearside wheel) 2L. For this purpose, the central processing unitcalculates a target value P1L for the fluid pressure to be introducedinto the wheel cylinder 5L provided for the left front wheel 1L asP1L=P1 where P1 is the sensed fluid pressure introduced into the wheelcylinder 5L and a target value P1R for the fluid pressure to beintroduced into the wheel cylinder 5R provided for the right front wheel1R as P1R=P1-ΔP. The central processing unit further calculates a targetvalue P2L for the fluid pressure to be introduced into the wheelcylinder 6L provided for the left rear wheel 2L as P2L=P3 where P3 isthe sensed fluid pressure introduced into the wheel cylinder 6L and atarget value P2R for the fluid pressure to be introduced into the wheelcylinder 6R provided for the right rear wheel 2R as P2R=P3-ΔP.

When the vehicle is turning to the right, the left front wheel 1L(offside wheel) is subject to a smaller braking force than the rightfront wheel (nearside wheel) 1R and the left rear wheel (offside wheel)2L is subject to a smaller braking force than the right rear wheel(nearside wheel) 2R. For this purpose, the central processing unitcalculates a target value P1R for the fluid pressure to be introducedinto the wheel cylinder 5R as P1R=P2 where P2 is the sensed fluidpressure introduced into the wheel cylinder 5R and a target value P1Lfor the fluid pressure to be introduced into the wheel cylinder 5Lprovided for the left front wheel 1L as P1L=P2-ΔP. The centralprocessing unit further calculates a target value P2R for the fluidpressure to be introduced into the wheel cylinder 6R as P2R=P4 where P4is the sensed fluid pressure introduced into the wheel cylinder 6R and atarget value P2L for the fluid pressure to be introduced into the wheelcylinder 6L provided for the left rear wheel 2L as P2L=P4-ΔP.

At the point 216 in the program, the central processing unit calculatestarget values for the current signals i1, i2, i3 and i4 applied to therespective pressure control valves 13F, 14F, 13R and 14R. Thesecalculations are made based upon the calculated target values P1L, P1R,P2L and P2R. The central processing unit also a target value for thecurrent signal i5 applied to the solenoid change-over valve 18. At thepoint 214 in the program, the calculate target current signal values aretransferred to the input/output control unit. The input/output controlunit sets the current signals i1, i2, i3 and i4 to cause the pressurecontrol valves 13F, 14F, 13R and 14R to control the fluid pressures tothe wheel cylinders 5L, 5R, 6L and 6R to the calculated target valuesP1L, P1R, P2L and P2R, respectively. In the illustrated case, theinput/output control unit maintains the current signals i1 and i3 asthey stand and takes the current signals i2 and i4 in the form of on-offsignals to reduce the fluid pressures P2 and P4 when the vehicle movesalong a left-handed cornering path and it maintains the current signalsi2 and i4 as they stand and takes the current signals i1 and i3 in theform of on-off signals to reduce the fluid pressures P1 and P3 when thevehicle moves along a right-handed cornering path. The input/outputcontrol unit employs the fluid pressure sensors to provide feedbackcontrols of the fluid pressures P1, P2, P3 and P4. The input/outputcontrol unit produces a current signal i5 having a set level to thesolenoid change-over valve 18. After the calculated values aretransferred to the input/output control unit, the program proceeds tothe end point 228.

If the answer to the question inputted at the point 208 is "no", then itmeans that the brake pedal 3 is released and the program proceeds toanother determination step at the point 218. This determination is as towhether or not it is a time at which the automatic braking mode isrequired. If the answer to this question is "yes", then it means thatthe vehicle is in a predetermined turning condition and the programproceeds to the point 220 where the current signal i5 is set at twoamperes in order to energize the solenoid change-over valve 18 so as toplace the braking system in an automatic braking mode.

At the point 222 in the program, a fluid pressure difference value ΔP iscalculated as a function of steering angle θ and steering speed θ'. Forthis purpose, the central processing unit calculates a basic fluidpressure difference value ΔP1 from a relationship programmed into thecomputer. This relationship specifies the basic fluid pressuredifference value ΔP1 as a function of steering angle θ. One example ofsuch a relationship is shown in FIG. 3 where the basic fluid pressuredifference value ΔP1 is zero when the steering angle θ is equal to orless than a predetermined value θ1 and it increases as the steeringangle θ increases when the steering angle θ is greater than thepredetermined value θ1. A correction factor K1 is used to modify thecalculated basic fluid pressure difference value ΔP1 so as to obtain thefluid pressure difference value ΔP as ΔP=K1×ΔP1. The correction factorK1 is calculated from a relationship programmed into the computer. Thisrelationship defines the correction factor K1 as a function of steeringspeed θ'. One example of such a relationship is shown in FIG. 4 wherethe correction factor K1 increases as the steering speed θ' increases.The calculated fluid pressure difference value ΔP corresponds to adifference between the fluid pressures to be introduced into the wheelcylinders 5L and 5R provided for the respective front wheels 1L and 1Rand also to the difference between the fluid pressures to be introducedinto the wheel cylinders 6L and 6R provided for the respective rearwheels 2L and 2R.

At the point 224 in the program, target values P1L, P1R, P2L and P2R forthe fluid pressures to be introduced into the wheel cylinders 5L, 5R, 6Land 6R provided for the respective wheels 1L, 1R, 2L and 2R arecalculated. These target values are calculated in such a manner that thefluid pressures (Pout) to be introduced into the wheel cylindersprovided for the outer or offset wheels positioned on the outside of acircle in which the vehicle moves are set at zero and the fluidpressures (Pin) to be introduced into the wheel cylinders provided forthe inner or nearside wheels positioned on the inside of the circle areset at the calculated fluid pressure difference ΔP.

For example, when the vehicle is turning to the left, the centralprocessing unit sets zero for a target value P1R for the fluid pressureto be introduced into the wheel cylinder 5R provided for the right frontwheel 1R and ΔP for a target value P1L for the fluid pressure to beintroduced into the wheel cylinder 5L provided for the left front wheel1L. The central processing unit further sets zero for a target value P2Rfor the fluid pressure to be introduced into the wheel cylinder 6Rprovided for the right rear wheel 2R and ΔP for a target value P2L forthe fluid pressure to be introduced into the wheel cylinder 6L providedfor the left rear wheel 2L.

When the vehicle is turning to the right, the central processing unitsets zero for a target value P1L for the fluid pressure to be introducedinto the wheel cylinder 5L provided for the left front wheel 1L and ΔPfor a target value P1R for the fluid pressure to be introduced into thewheel cylinder 5R provided for the right front wheel 1R. The centralprocessing unit further sets zero for a target value P2L for the fluidpressure to be introduced into the wheel cylinder 6L provided for theleft rear wheel 2L and ΔP for a target value P2R for the fluid pressureto be introduced into the wheel cylinder 6R provided for the right rearwheel 2R. Following this, the program proceeds to the point 214.

If the answer to the question inputted at the point 218 is "no", thenthe program proceeds to the point 226 where the fluid pressuredifference ΔP is set at zero. Following this, the program proceeds tothe point 224. In this case, neither vehicle turning behavior controlnor no automatic braking operation is performed.

Although the fluid pressure difference value ΔP has been described ascalculated as a function of steering angle θ and steering speed θ', itis to be understood that the fluid pressure difference value ΔP may becalculated as a function of steering angle θ, steering speed θ' andvehicle speed V. In this case, the fluid pressure difference value ΔP iscalculated as ΔP=K1×K2 X ΔP1 where K2 is a correction factor calculatedfrom a relationship which specifies the correction factor K2 as afunction of vehicle speed V. One example of such a relationship is shownin FIG. 5 where the correction factor K2 decreases from 1.0 as thevehicle speed V increases. The vehicle speed V may be calculated basedupon the speeds of the driven wheels of the vehicle.

In this modification, the vehicle turning behavior control is madeduring an automatic braking mode when the vehicle is turning either tothe left or to the right. The difference between the braking forcesapplied to the offside and inside wheels is corrected for the steeringspeed θ' in a direction to increase the yaw moment as the steering speedθ' increases.

Although the vehicle turning condition is detected based upon thesteering angle θ, it is to be understood that the yaw rate or thelateral acceleration g sensed by the lateral acceleration sensor 29 maybe used singly or in combination with the steering angle θ.

Although the invention has been described in connection with a vehicleturning behavior control apparatus arranged to provide a differencebetween the braking forces applied to the respective front wheels andbetween the braking forces applied to the respective rear wheels, it isto be understood that the vehicle turning behavior control apparatus maybe arranged to provide a difference between the braking forces appliedto the respective front wheels or between the braking forces applied tothe rear wheels.

In addition, the fluid pressure difference ΔP may be obtained as afunction of time. In this case, the fluid pressure difference ΔPgradually decreases to zero as time progresses after the fluid pressuredifference ΔP is calculated once.

Although the fluid pressure sensors 31L, 31R, 32L and 32R are used toprovide feedback control of the fluid pressures to the respective wheelcylinders 5L, 5R, 6L and 6R, it is to be understood that the vehicleturning behavior control apparatus may be arranged to provide an openloop control for these fluid pressures. In this case, the fluid pressuresensors may be removed.

What is claimed is:
 1. An apparatus for controlling turning behavior ofa multi-wheeled automotive vehicle supported on a plurality of pairs ofwheels, the apparatus comprising:braking means for applying brakingforces to the respective wheels; first sensor means sensitive to avehicle steering angle for producing a first signal indicative of asensed vehicle steering angle; second sensor means sensitive to avehicle steering speed for producing a second signal indicative of asensed vehicle steering speed; and a control unit coupled to the firstand second sensor means, the control unit including, means forcalculating a basic value ΔP1 as a function of the sensed vehiclesteering angle to increase the basic value as the sensed vehiclesteering angle increases when the sensed vehicle steering angleincreases when the sensed vehicle steering angle exceeds a predeterminedvalue, means for calculating a first correction factor K1 as a functionof the sensed vehicle steering speed and to increase the firstcorrection factor as the sensed vehicle steering speed increases, meansfor calculating a second correction factor K2 as a function of vehiclespeed and to decrease the second correction factor as the vehicle speedincreases, means for calculating the difference ΔP as ΔP=ΔP1×K1×K2,thereby reducing the difference ΔP between the nearside and offsidewheels as the vehicle speed increases, and means for setting the brakingmeans to provide the calculated difference ΔP between the braking forcesapplied to nearside and offside wheels of at least one of the pairs ofwheels so that the braking force applied to the offside wheel is smallerthan the braking force applied to the nearside wheel.
 2. The apparatusas claimed in claim 1, further comprising third sensor means associatedwith a brake pedal of the vehicle for producing a third signal when thebrake pedal is depressed, and wherein the control unit includes meansresponsive to the third signal for causing the braking means to set thebraking force applied to the nearside wheel at a first valuecorresponding to an amount the brake pedal is depressed and the brakingforce applied to the offside wheel at a second value equal to the firstvalue minus the calculated difference.
 3. The apparatus as claimed inclaim 2, wherein the braking means includes automatic braking meansoperable for applying constant braking forces to the respective wheelswhen the sensed vehicle steering angle exceeds a predetermined value,and wherein the control unit includes means operable, when the automaticbraking means is operating, for causing the braking means to set thebraking force applied to the offside wheel to zero and the braking forceapplied to the nearside wheel at a value equal to the calculateddifference.